Where were they? It was a question that plagued me during my anxiety-ridden dives in the summer of 2008, as I paddled through the bathlike waters of Tampa Bay, Fla., hoping to find enough seahorses to complete my graduate degree in evolutionary biology. After a month and a half of fruitless searching, I finally threw in the rag and headed back to Indiana University with the dreaded “failed field season” added to my resume. But it wasn’t just a disappointment, it was a surprise—just 2 years earlier, working in the same location, I had captured, measured, and tagged 73 animals in just 6 weeks. Where did they go? And, more important, why?

The Florida Fish and Wildlife Conservation Commission (FWC) has been monitoring the fish of Tampa Bay since the late 1980s. Sure enough, their numbers confirmed my experience: Between 2006 and 2008, the number of lined seahorses (my chosen organism) surveyed by the FWC dropped by more than 50 percent. “It is very strange,” says ichthyologist Ed Matheson of the FWC’s Fish and Wildlife Research Institute. “I can’t think of any explanation other than, and this is really speculative, the big red tide in 2005.”

Red tides are algal blooms of the dinoflagellate Karenia brevis, which produces highly potent neurotoxins that can be lethal to a wide range of fish species. The bloom of 2005 was particularly devastating. “Typically the red tide is more coastal,” explains fish ecologist Bob McMichael of the FWC. “This time it came into Tampa Bay and hit the grass beds”—the spawning sites of many large game fish and the sheltering home of seahorses and pipefish, their uncurled cousins. The intensity of the 2005 bloom may have stemmed from four hurricanes that hit Florida in 2004, which nearly doubled the normal rainfall—dumping higher levels of nutrients into the bay and fueling the algal growth (Geophys Res Lett, 33:L11601, 2006).

But the lined seahorse populations didn’t drop until 3 years after the red tide hit the bay. Delving further back in the survey data reveals a stark increase in 2006—5 times the 2005 collections. “Maybe—and that’s a big maybe—[the tide] killed off a lot of their predators, [which] allowed [the seahorses] to build up a larger population,” Matheson speculates.

Indeed, in the 2 years following the red tide, many fish species—including red drum and spotted sea trout—declined dramatically. Thus, the dramatic drop in seahorse numbers in 2008 may reflect an overall ecosystem shift, with the recovering predator populations bringing the inflated seahorse numbers back into check.

Physical factors, such as short-term temperature variations or overall rainfall—which influences both the salinity and dissolved oxygen content of the bay—could also affect species abundances. “The thing that’s frustrating is that it’s hard to define a normal year,” Matheson says. “Something changes every year.”

Furthermore, increasing water temperatures as a result of global warming may affect fish species’ watery world, says University of Tampa researcher Heather Masonjones, who has been surveying seahorses and pipefish in the bay since December 2005. “As the temperatures increase in the summer, the shallow seagrasses where they typically spend their time are going to become inhospitable.” As the seahorses head to the cooler refuge of deeper water, they are not finding suitable habitat, as deeper waters lack the sunlight necessary to sustain the grass beds.

“But it’s all conjecture [at this point],” says Masonjones. “There’s not a whole lot we can say with any certainty at all. [And] unfortunately, by the time we get to that point, there’s often very little we can do to fix the situation.”

After those 6 frustrating weeks in field, I bid goodbye to the missing seahorses and the dissertation they were to support. Instead, I found my calling in science journalism. I composed a Master’s thesis on the basis of my laboratory experiments on seahorse mating behavior and accepted a position at The Scientist the following summer.

I used an injectable polymer to tag the seahorses. Basically, it's a tiny injection of colored plastic at the base of the tail that shows through their relatively transparent skin. In this way, I could create different color combinations to identify individuals. Studies have shown that this method does not hinder motility or increase mortality in any way.

Has it been proven that seahorses can survive the manner in which they are tagged? What if tagging them is in fact causing their demise?